This invention relates to oscillator circuits, and more particularly, to tunable voltage controlled oscillator circuits.
A variety of applications utilize voltage controlled oscillators (VCOs). For example, wireless radio communications systems generally transmit voice and/or other data between transceivers, which may be fixed and/or mobile radio communications terminals, via the propagation of radio frequency (RF) electromagnetic waves. Such applications desirably utilize stable and accurate circuits for generation of oscillating electrical signals used in creating signals in a form suitable for transmission over the wireless system. One circuit widely used to generate such oscillating signals is a phase-lock loop (PLL). A PLL is an electrical circuit that may be used to generate an oscillating output signal that has a constant phase relationship with an oscillating input signal. By utilizing a highly stable and accurate source, such as a crystal oscillator, to generate the oscillating input signal, and various frequency multipliers and dividers, a stable and accurate oscillating output signal can typically be generated across a range of frequencies.
One component of a typical PLL is a VCO. The VCO typically generates an oscillating signal at its output, the frequency of which is responsive to a voltage level applied at its input. In the PLL, the voltage input, referred to herein as the tuning voltage, may be a function of the phase error between the output of the VCO and the desired oscillating signal. The VCO thus may generate an oscillating signal at a frequency that varies over a finite range, corresponding to variations in the tuning voltage over a corresponding finite range.
The specific parameters of this voltage/frequency relationship may depend upon the design of the VCO, the values of electrical components that comprise the VCO, ambient temperature, and other effects known in the electronics arts. Ideally, if plotted on a voltage/frequency axis, the relationship would define a generally linear operating curve with either a positive slope (i.e., an increase or decrease in the tuning voltage causes a corresponding increase or decrease in the frequency of the oscillating signal generated by the VCO) or a negative slope over a xe2x80x9clinearxe2x80x9d operating range. It is to be understood that a non-linear relationship between the input voltage and the output frequency of the VCO may, in practice, be provided at frequencies above and/or below the generally linear operating range.
To expand the frequency range of a VCO, it is known to selectively couple frequency altering components, such as, capacitors, varactors, FET transistors, and the like, to the resonant circuit in the VCO. This alters the range of frequencies generated in response to the tuning voltage, in effect establishing a new linear operating curve for the VCO. For example, it is known to couple various capacitors to a VCO via a programmable switching matrix such that, by selectively configuring the switches, a plurality of overlapping linear ranges for the VCO may be selected. This ensures that the VCO may be calibrated to compensate for deviations in its linear range due to manufacturing process variations or other parasitic effects, by choosing a VCO operating curve to encompass the desired range of VCO operation. This calibration, also know as VCO trimming, generally occurs in the factory upon manufacturing of the integrated circuit containing the VCO, such as, for example, by burning fuses or by programming a particular value in a register, the contents of which control the switches connecting the transistors to the VCO oscillator. Such a calibration is generally not suited to self-tuning during operation. Alternatively, a complex control relationship based on a plurality of inputs may be used to allow tuning of a VCO in the field rather than at the factory as described, for example, in U.S. Pat. No. 5,648,744.
Another known approach to improving performance of a phase lock loop including a VCO is aided acquisition. Aided acquisition may be added to a phase lock loop (PLL) such as a PLL, utilizing a simple phase detector as an error signal (tuning voltage) generator, as such circuits may have problems with locking under various conditions. An aided acquisition type PLL circuit is illustrated in FIG. 1. As shown in FIG. 1, the PLL 100 includes a VCO 110 which receives a tuning voltage input 130 generated by a phase detector 120. A mixer 140 receives a local oscillator (Lo) signal to provide a feedback circuit from the output of the VCO 110 to the phase detector 120. The PLL 100 shown in FIG. 1, thus, provides a circuit for generating a radio frequency (RF) output signal in response to an intermediate frequency (IF) input signal to the phase detector 120 and the local oscillator reference frequency. It is to be understood that the PLL 100 may include other components, such as filter circuit components, not illustrated in FIG. 1.
The VCO 110 generates the radio frequency (RF) output signal responsive to the tuning voltage 130. The mixer 140 combines the radio frequency output signal with the local oscillator signal to generate an intermediate frequency signal that is compared by the phase detector 120 to the intermediate frequency input signal. Thus, the phase detector 120 generates an output signal whose voltage is dependent on the phase relationship between its two inputs to provide a tuning voltage signal 130 used to drive the VCO 110.
Aided acquisition is provided to the circuit of FIG. 1 by inclusion of a sweep voltage generator 150. The sweep voltage generator 150 typically injects a constant current into the circuit of the PLL 100 on the tuning voltage 130 to generate a frequency sweep by the VCO 110. The sweep voltage generator 150, thus, facilitates locking of the PLL 100 to a desired frequency.
The aided acquisition circuit of FIG. 1 may still be subject to problems with obtaining yield during manufacturing at a desirable phase noise performance level for an integrated VCO type PLL. It is known that the range of frequencies to which the PLL 100 of FIG. 1 may be tuned may depend on the characteristics of the VCO 110. In particular, a trade-off in the design of voltage controlled oscillators is typically the interplay between tuning range and phase noise characteristics of the voltage controlled oscillators due to the potentially limited resolution of the tuning voltage input signal. Thus, by restricting the tuning range of the VCO 110, phase noise may be reduced as fluctuations in the tuning voltage will generate narrower corresponding fluctuations in the output frequency from the VCO 110. However, the resultant frequency range of operation of the PLL 100 may be unsuitable for certain applications.
In embodiments of the present invention, voltage controlled oscillator circuits are provided including a voltage controlled oscillator (VCO) having an input and an output responsive to the input. A tuning circuit coupled to the VCO sets a relationship between the input and the output of the VCO. An aided acquisition circuit is coupled to the input of the VCO. A control circuit selects a state of the tuning circuit to set the relationship between the input and the output of the VCO. The control circuit also controls operation of the aided acquisition circuit responsive to changes in the state of the tuning circuit. Methods for operating voltage controlled oscillator circuits are also provided. In addition, phase lock loop circuits and mobile terminals including the voltage controlled oscillator circuits are provided.